I'm trying to fix it myself, and no but i was assured it worked properly, i'm contacting the seller about it now.

Anyways... I think i have found the source of my issues:

Here is a picture of it jamming or whatever. The bolt is at half-cock position but the nozzle is far forward and almost a bit wedged in/hard to pull back.

Why is this occuring? It doesn't happen every round but rather every few, at least. If i manually cock it back at that point the gun would proceed to fire correctly.

The gun, when set like above, makes a weird noise and only partially blows-back, not enough to catch the sear so it will shoot a short burst of gas that varies from just dry-firing/wasting some gas, to dumping the whole magazine's load of gas out at once.

Okay so as i thought the lube was simply blown out of the system after the first mag and a half, and now it's back to it's old self.

Either the cylinder or piston head seal is at fault here. the Cylinder/loading nozzle has a MK11 marking on it and the seal is green but clearly puts too much pressure on the walls of the cylinder except when coated with silicon oil.

remember there's also KSC m11a1 system 7 made in japan. Those cylinder are thinner and only made for HFC134 gas and both marked with mk11. The m11 system 7 Japan parts are different from system 7 Taiwan version. What if the seller lied and told you he bought it from here, but really he bought it from some website.

Okay guys, been a while, but update.
Borrowed a friend's MAC.
I swapped our cylinder seals (the green piston head bit)
His gun works fine with my seal.
Mine vents gas instantly on the first shot. Gas even shoots out of the two stock tang holes! (dont have stock installed)